The present invention relates generally to sensors based on optical fibers, and more particularly to active and passive sensors utilizing chiral fiber elements.
In recent years, passive sensors based on fiber optic elements have found increased use in various industrial and commercial applications. Fiber optic sensors typically utilize a fiber Bragg grating element having predefined stop band gap (reflection band) characteristics centered around a pre-defined wavelength range. A broadband emission source or a continuously tunable laser emit radiation toward the fiber Bragg grating element over a broad range of wavelengths, while a photodetector detects wavelengths reflected by the reflection band. Changes in pressure or temperature (depending on the sensor configuration) cause the reflection band to shift. Thus, because the photodetector monitors the shifts in the wavelengths reflected or transmitted through the fiber Bragg grating, the fiber optic sensor, depending on its configuration, can detect changes in pressure or temperature.
The above description refers to previously known passive sensors. Attempts have been made to design an active sensor based on a fiber Bragg grating. An active sensor requires optically pumping a fiber Bragg grating doped with optically excitable material to produce lasing at a predefined wavelength and then detecting shifts in the wavelength due to temperature or pressure. The advantage of an active sensor is that a broadband (or a continuously tunable) emission source is not required. However, due to the nature of previously known fiber Bragg gratings, a fiber Bragg grating would need to be of significant length (e.g. at least approximately 15 meters) in order to produce lasing, making an active sensor impractical.
Finally, both types of previously known fiber optic sensors suffer from a number of drawbacks. Fiber Bragg gratings are typically manufactured through irradiating, with UV light, an optical fiber made from a UV-sensitive material through a pre-designed phase mask. Another prior-art approach to manufacturing fiber Bragg gratings involves irradiating a UV-sensitive optical fiber with two interfering UV laser beams. In both approaches, the resulting fiber Bragg gratings have two significant disadvantages. First, the requirement that the optical fiber be UV-sensitive limits the application of the prior art sensors in that the sensors cannot be used in an environment in which exposure to UV radiation may occur. Second, the UV-sensitive fiber requirement limits the choice of materials used in fabricating the fiber Bragg gratings. In addition, while previously known fiber Bragg gratings are sensitive to shifts in temperature and pressure, they are not sensitive to axial twisting. Finally, the previously known fiber Bragg gratings are relatively expensive and difficult to manufacture.
It would thus be desirable to provide an optical fiber based sensor that can be used in locations exposed to UV radiation. It would further be desirable to provide an optical fiber based sensor that can be fabricated from any material suitable for optical fibers. It would additionally be desirable to provide an optical fiber based sensor that is responsive to axial twisting. It would also be desirable to provide an active optical fiber based sensor that is small and practical to use. It would further be desirable to provide an optical fiber bases sensor that is easy and inexpensive to manufacture.